Heavy medium for gravity separations



United States Patent 3,021,282 HEAVY MEDIUM FOR GRAVITY SEPARATIONSEverett L. Joppa, Duluth, Minn., assignor to P-M Associates, Cleveland,Ohio, a partnership No Drawing. Filed Sept. 4, 1958, Ser. No. 759,049 2Claims. (Cl. 25260) This invention relates to the benefication art andmore particularly to improvements in processes of concentrating orup-grading mineral values by procedures involving use of heavy media,e.g. the float-sink process of separating coal from slate, thehydrocyclone separation of iron ore from gangue material, and the like.The invention is concerned with the provision of a novel heavy mediumfor use in such gravity separation procedures, of a solid component ofsuch heavy medium, and of a process of preparing such solid component.

As solid components of heavy media there have been used a variety offinely divided mineral solids. From the standpoint of recovering thesolid component of the heavy medium from the float-sink products (or,from the underflow and overflow products of the hydrocycloneseparation), it is particularly advantageous if said solid component ismagnetic. Thus, it heretofore has been proposed to use as the solidcomponent or medium of the heavy medium the product obtained by crushingand grinding magnetite and screening out the particles larger than somearbitrary upper limit of size. The heavy medium wherein such sizedmagnetite crystals compose the solid constituent is operable, but itsuse is circumscribed by the limiting factor as to the specific gravityof the resulting heavy medium of usable viscosity characteristics.Ordinary crushed magnetite breaks into irregular, angular shapedparticles upon grinding. As is known, the viscosity of a pulp made of asuspension of angular particles in a liquid is greater than that of asimilar suspension of rounded particles (U.S. Bureau of Mines, Report ofInvestigation 3469-R, DeVaney and Shelton). Since a goal in developingheavy media is to provide a pulp which is as heavy as possible whilehaving an acceptably low viscosity, the relatively high viscosity of apulp of angular magnetite particles materially limits" use of the same.

For improving the specific gravity characteristic of a heavy medium fromground magnetite without unduly increasing its viscosity it has beenproposed to admix with the magnetite a greater or lesser amount offinely divided ferrosilicon. However, this latter is several times asexpensive as is natural magnetite, and such added expense rules out theuse of such mixtures in concentrating procedures applied to verylow-cost materials.

It has been found that when powdered coal is burned, in a modern powerplant, the iron initially present in the coal is converted to a magneticmineral which is largely magnetite but contains some iron silicates. Theparticles of the fly ash so formed are spherical in shape due to thefusion of the coal ash while in a state of suspension. The magnetitecontent of these fly ash particles is characterized by being extremelyfine grained, many of the crystals of which are sub-micron in size, thecrystals being encased in and bound together with a slag bond intospheres which strongly resist abrasion and tend to remain spherical inuse.

From such fly ash (i.e., the finely divided solid residue from thecombustion of powdered coal) there may be recovered, in the stateformed, a distinctly magnetic fraction, composed of particles all ofwhich are finer in size than some predetermined maximum and practicallyall of which are true spheres, which fraction contains more than 53% byweight of iron mostly as magnetite and less than 16% silica, whichmagnetic fraction can be suspended in water or other suitable aqueousliquid to provide an excellent heavy medium for use in the cycloneseparation of iron ore from gangue or in the float-sink process ofup-grading coal or similar gravity separation applications. Thespherical shape of the magnetic fly ash particles makes this concentratea particularly valuable material for the purpose stated, because of thematerially lower viscosity of their suspension as contrasted with theviscosity of a similar suspension of irregular, angular shaped particlesof ground magnetite. Moreover, their extraordinary resistance toabrasion is in contrast to disintegrated natural magnetite, which latteris only a medium hard mineral and which, because of a normally coarsecrystalline nature, tends to break down (in use) rather easily, givingundesirable magnetite slimes. These slimes inherently increase theviscosity of the system and tend to increase the media loss whichresults from the circumstance that it is difiicult for magneticseparators to recover efficiently very fine material (5 microns andfiner). Because of the exceedingly extensive surface present in theslime particles the latter tend readily to oxidize tohighernon-magneticoxides such as hematite and, hence, are lost in anensuing magnetic recovery step.

The heavy medium component of the present invention has amongst others,the following advantageous characteristic: From the standpoint ofeconomics, an outstanding characteristic is cheapness, being recoveredfrom a waste product of steam power plants by a simple and inexpensivemethod which avoids all crushing and grinding. Secondly, the trulyspherical form of practically all of the particles constituting themagnetic fractionruakes it possibleas compared with ground naturalmagnetite-to produce a higher specific gravity heavy medium at the sameviscosity (or, in the obverse, to produce a lower viscosity heavy mediumat the same specific gravity). The magnetic fraction is more nearlyrecoverable (from the separation products) than is ground naturalmagnetite.

It was discovered that grinding this fly ash'either before or aftermagnetic separation, destroys its usefulness. This is in direct contrastto what might normally be expected. If the material is ground prior tomagnetic separation, the desirable spherical nature of the material isdestroyed and surprisingly the resultant concentrate contains less ironand more silica which in itself is an undesirable attribute. This comesabout because the lower iron content spheres, upon grinding andsubjecting to magnetic concentration, yield a percentage of a very lowgrade iron concentrate that dilutes the remainder of the concentrate.With grinding before concentration, the iron unit recovery is higher,but the product then is inferior both as to grade and as to particleshape. If grinding is pract ced before concentration, the particle shapeis then no better than a nautral ground magnetite. If grinding ispracticed after the initial magnetic concentration step, the desirableparticle shape is likewise destroyed.

The magnetic susceptibility of the magnetic mineral recovered from flyash is in the same order as that secured from natural magnetite ores.Pure magnetite has a relative attractability of approximately 40.00using as a scale the measurements proposed by Mr. E. W. Davis in theMinnesota School of Mines Bulletin No. 7. This scale is based on theassumption that pure iron has an attractability of 100. Most magnetitesavailable for commercial use in the heavy media process, because ofattached gangue, have an attractability of approximately 35. Inseparating magnetite from fly ash we purposely use magnetic separatorswhich have a field strength equal to 400 gauss 2 inches from the drum,as contrasted to separators used for recovering magnetite from themedium which have a strength of approximately 600 gauss at the samedistance from the drum. This is done so as to discard any of the verylean spherical magnetic particles which would tend to lower the grade ofthe magnetite recovered. It also insures that only a minor amount ofsuch magnetite will be lost in the separators used to clean up the mediain the sink-float or cyclone processes. Tests on various samples of flyash magnetite indicate that the attractability is in the range of from30' to 36 on the scale just mentioned. For comparative purposes itshould be mentioned that commercial separators with a strength of 700gauss 2 inches from the pole will recover materials with a magneticattractability as low as 10.

For producing the solid component of a heavy medium in accordance withthe present invention, the fly ash iswithout being crushed, ground orotherwise broken uppreliminarily screened at say, 8 mesh and theoversize is discarded. The minus 8 mesh portion is then subjected to amagnetic separation procedure and the non-magnetic portion is discarded.Finally, from the magnetic portion there is separated a fraction all ofthe particles of which are finer than a predetermined maximum, and theparticles larger than said maximum are discarded. The so-obtainedfraction is suspended in aqueous liquid to provide the novel improvedheavy medium of the present invention.

By the use of the expression predetermined maximum reference is heremade to the fact that the maximum size of particle selected is relatedto the particular beneficiation application under consideration. Where,as in the hydrocyclone beneficiation of low-grade iron ore maat'efials,a relatively coarse medium is of advantage, the fpredeter'mined maximumsize of medium may be as coarse as 35 'mesh. Atthe other extreme,where-as in the tip-grading of coal-float-sink separators of thenon-setting type are sometimes used and a relatively fine rnedium is'desired, the predetermined maximum size of particle maybe as fine as100 mesh. In either event, the'frac'tion is composed of sphericalparticles of a variety of sizes ranging from the aforesaid predeterminedmaxiinner to particles finer than 325 mesh, the greater part of thetotal being minus 325 mesh.

Three fly ashsamples, from as many sources, were examined. The followingstructure analysis data were secured:

' STRUCTURE ANALYSIS Fly ash samples, as received Sample ".A] Sample 13Sample Size Percent Accm. Percent Accm. Percent Acorn.

Wt. Wt. Wt.

Total 100. 00 100. 00 100. O0 100. O0 100. 00 100. 00

' SampleA 18.57 Sample 26.79 'Sample"C- 23.66

4 with varying amounts of silica, alumina, lime, magnesia, manganese,phosphorus and sulphur.

The magnetic mineral recovered from fly ash has a truly spherical shape.

Chemical examination of the samples showed that the finer the particlethe higher was the iron content and the lower was the silica content.The following data, pertinent to sample B, are generally illustrative ofall three samples.

SCREEN ANALYSIS OF SAMPLE 13" 8 mesh magnetic separator concentratesAssay, percent Size Percent Iron Silica 3. 61 l 25. 31 2. 25. 47 2. 4725. 07 2. 08 25. 1. 70 26. 44 1. 41 28. 53 42. 52 l. 28 35. 30 36. 41 l.16 46. 91 22. 93 1. 50 49. 81 20. 44 3. 51 53. 68 16. 36 4. 88 57. O612. 90 24. 36 59. 96 10. 07 49. 24 63. 83 7.00

Total 100. 00 56. (19

Composzte results 48 Mesh FractionunQ 85.93% Wt.-." 61.04% Iron, 9.47%Silica. 65 Mesh Fractiou 84.65% Wt 61.43% I'ron, 9.07% Silica. -100 MeshFraction 81.99% Wt 61.84% Iron, 8.66% Silica.

The aforesaid magnetic con'centratejof fly ash com-- pared with groundnatural magnitite, as to structure, grade and specific gravity, asfollows:

FLY ASH MEDIA vs. GROUND MAGNETITE STRUCTURE, GRADE AND SPECIFIC GRAVITYGround Size Fly Ash Magne tlte 65 1.49 -e 20M 1. 35 100M 1. 75 2. 43 M4.08 4. 67 znnM 5. 68 7.48 325M 28. 35 18. 30 --325M 57. 30 67. 12

Total 100. 00 100. 0D

61. 04 64. 09 9. 47 9. l8 Specific Gravity 4. 39 4. 53

The invention will now be described in'greater particularity withreference to the following illustrative specific examples.

EXAMPLE 1 In this example are shown the results of making a floatsinkseparation on minus 4 inch plus one-fourth inch bituminous coalcontaining shaley impurities.

In one case typical ground magnetite containing approximately 2.4% byweight coarser than 100 mesh and 67% finer than 325 mesh was used asmedium. In the comparative tests, spherical magnetite from fly ash wasmade by taking furnace ash, rejecting the +8 mesh material andmagnetically separating the undersize and again rejecting the portion ofthe concentrates coarser than 48 mesh. In the two comparative tests thespecific gravity of the medium was held at the same point, namely, 1.45.Because of the fact that 'thefflyash magnetite was spherical it had aconsiderably lower vis cosity than had the identical suspension made upof ground natural magnetite. This lower viscosity permitted a moreclearcut separation between coal and shaley impurities to be made. Themagnetite from the fiy ash, because of its shape, could be morecompletely removed and recovered in the screening and magneticconcentration stages of the recovery processes than was the normal typeof ground magnetite. These results are shown in the accompanying table,where it may be seen that the magnetite consumption dropped from 1.05#per ton of feed to .35# per ton of feed. It may also be seen that theuse of this special magnetite resulted in a sharper separation of thecoal from the waste, and also made it In certain geographical areas itis sometimes diflicult to find gravel, that is sufficiently free fromundesirable material, such as shale, that can be used as aggregate togive a concrete of high strength. In recent years it has become rathercommonplace to treat such gravel with a heavy media process. For such anoperation a gravity in the order of 2.45 is necessary to separate thesofter and lower density shales and like materials from the denserconstituents of gravel. Most ground magnetites have a density ofapproximately 4.8. With such magnetites it is possible to secure asuspension close to the desired density if the media is perfectly cleanand there is no slime contamination. Under normal conditions, however,the natural magnetite suspensions are viscous and it is difiicult tomaintain a sufiiciently high gravity to make an effective separation.Many operators have therefore found it expedient, in order to secure asufficiently high gravity together with a reasonable viscosity, to use amixture of ferrosilicon and magnetite. This results in considerableextra expense since ferrosilicon normally costs five times as much asmagnetite. It has been found that the magnetite produced from fly ashbecause of its spherical nature will have, at the same densities, aconsiderably lower viscosity than if the regular ground natural type ofmagnetite is used. It is therefore possible to use this fly ash type ofmagnetite alone without the necessity of ferrosilicon additions for thispurpose.

EXAMPLE 2 This example summarizes test work on a sample of gravelrequiring upgrading. In one case a mixture of ground magnetite andferrosilicon was used and in the other the preferred fly ash magnetitewas employed. The pulps were held at the same specific gravity. With thepreferred type of magnetite, the amount of misplaced material in theaggregate of sink products was only 1%. Where a mixture of groundmagnetite and ferrosilicon was used, the amount of such misplacedmaterial was 1.5%. As in Example 1, the media loss using the preferredmaterial was considerably less than for the mixture of natural magnetiteand ferrosilicon. This substitution of media represented a saving of 2per ton of crude material which, with a low value product such asgravel, amounted to a very substantial saving.

The comparative results obtained are shown in Table 2 following:

6 TABLE 2 Float-sink separation of gravel for concrete aggregate The useof hydrocyclone has come into general use in concentrating fine sizes ofiron ores as well as some other minerals. On the Mesabi Range inMinnesota it is widely used for treating the washed portion of the ore.In such operations the media is usually made up of ground magnetite andto a lesser degree, magnetic sinter fines. The preferred size of suchmatrial is usually -48 mesh. Because part of the separating effect isdue to centrifugal force as well as to the density of the medium, it isnot necessary to employ as dense a medium as is utilized in thesink-float separation of such ores. Normally a pulp is maintained thathas a specific gravity of from 2.30 to 2.40. Example 3 gives the resultsof a cyclone separation test made on an ore of the type described.Examples are shown of the results using ground magnetite as compared tothe magnetite made from fly ash. Because of the lower viscosity possiblewith a preferred variety of magnetite it was possible to maintain aspecific gravity of media of 2.45 compared to the usual 2.38 and stillmaintain a workable viscosity and this resulted in producing a superiorgrade of concentrate with a lower media consumption per ton of material.In this particular case the reduction in the amount of magnetiteconsumed amounted to 2.5# per ton of feed which represented a saving ofapproximately 4. On the basis of the concentrate produced, the savingamounted to 9.1. It will also be noted that, due to the higher gravityused in the cyclone operation, the iron content of the concentrate wasincreased from 54.46% to 56.72%. There was a corresponding reduction inthe silica content.

The comparative results obtained are set forth in Table 3 following:

TABLE 3 Cyclone separation of Mt" hematite ore fines 1. A heavy mediumfor use in the gravity separation of a mixture of solid mineralparticles of different specific gravities, said heavy medium consistingessentially of an aqueous liquid suspension of a magnetic fraction offly ash composed of generally spherical particles consisting essentiallyof magnetite and of varying sizes all of which are finer than 35 meshand the greater part of which are finer than minus 325 mesh particles,the particles of said magnetic fraction assaying more than 53% by weightof iron mostly as magnetite and less than 16% by weight 7 of silica,said heavy medium having a specific gravity within the range 1.45 and2.45.

2. A heavymedium for use in the gravity separation of a mixture of solidmineral particles of different specific gravities, said heavy mediumconsisting essentially of an aqueous liquid suspension of a magneticfraction of fly a'sh composed of generally spherical particlesconsisting essentially of magnetite and of varying sizes all of whichare finer than 35 mesh and the greater part of which are finer thanminus 325 mesh particles, the particles of said magnetic fractionassaying more than 60% by weight 'of iron mostly as magnetite and lessthan 10% by Weight of silica, the particles composing said fractionhaving a 8 specific gravity'inexc'ess of 4.0 and said heavy mediumhaving a specific gravity within the range 1.45 and 2.45.

References Cited in the file of this patent UNITED STATES PATENTS1,958,351 Shingshang May '8, 1934 2,122,236 Nichols etal. June 28, 19382,430,186 Rakowsky Nov. 4, 1947, 2,686,593 Vogel et a1 Aug. 17, 19542,774,734 Rodis et al.v Dec. 18, 1956 2,878,518 Klee Mar. 24, 1959

1. A HEAVY MEDIUM FOR USE IN THE GRAVITY SEPARATION OFA MIXTURE OF SOLIDMINERAL PARTICLES OF DIFFERENT SPECIFIC GRAVITIES, SAID HEAVY MEDIUMCONSISTING ESSENTIALLY OF AN AQUEOUS LIQUID SUSPENSION OF A MAGNETICFRACTION OF FLY ASH COMPOSED OF GENERALLY SPHERICAL PARTICLES CONSISTINGESSENTIALLY OF MAGNETITE AND OF VARYING SIZES ALL OF WHICH ARE FINERTHAN 35 MESH AND THE GREATER PART OF WHICH ARE FINER THAN MINUS 325 MESHPARTICLES, THE PARTICLES OF SAID MAGNETIC FRACTION ASSAYING MORE THAN53% BY WEIGHT OF IRON MOSTLY AS MAGNETITE AND LESS THAN 16% BY WEIGHT OFOF SILICA, SAID HEAVY MEDIUM HAVINGA SPECIFIC GRAVITY WITHIN THE RANGE1.45 AND 2.45.